I am not sure about the length of time, but I had recurring UTI's for about 6 years after my colon and bladder grew together and exchanged contents severely. Then I read an article about what is called biofilm. Apparently, some bacteria will create a film over them that keeps antibiotics from reaching them, i.e. biofilm. Once the coast is deemed to be clear they come out. The only way to get rid of them is to treat them with one type of antibiotic for several days, then switch to a different type of antibiotic. Once I told the doctor this, he was aware and started me on this regiment. It worked.